High-Strength, Lightweight Austenitic-Martensitic Steel and the Use Thereof

ABSTRACT

The invention relates to a high-strength, lightweight austenitic-martensitic steel and the use thereof. The inventive lightweight steel is characterized by a chrome content of more than 0.5% and less than 18%, a silicon content of more than 1% and less than 4%, a manganese content of more than 2.5% and less than 30% and an aluminum content of more than 0.05 to 4% and lies within an alloy range that is determined by the coordinates of four points (Cr equ =2; Ni equ =2), (Cr equ =2; Ni equ =24), (Cr equ =20; Ni equ =10) and (Cr equ =20; Ni equ =6.5), whereby the chrome and nickel equivalent is calculated from the chemical composition of the steel using the relations (1) and (2): Cr equ =% Cr equ +% Mo+1.5% Si+0.5% W+0.9% Nb+4% Al+4% Ti+1.5% Ni eqe =% Ni+30% C+18% N+0.5% Mn+0.3% Co+0.2% Cu−0.2% AI (2). The indications are in weight percent and remainder substantially consists of iron and other elements usually present in steel (P. S). The inventive steel can be cold-formed, and is suitable for use as a material for hot- and cold-rolled sheets, strips and tubes, for non-flat semifinished products and non-flat products and retaining elements, for crash-relevant components and reinforcing structural components in the automobile industry, for expendable parts and as a material for weatherproof, corrosion resisting and stainless parts.

The improvement relates to a high-strength austenitic-martensiticlightweight steel alloyed with chromium, silicon, manganese, andaluminum and having a tensile strength greater than 800 to 1,200 MPa andan elongation at break greater than 25%, and relates to its use.

Steels with tensile strengths above 600 MPa are referred to aslightweight steels because the tensile strength per weight unit ishigher than that of aluminum.

PRIOR ART

There are various possibilities for increasing the strength ofmulti-phase steels such as austenitic-martensitic steels. For example,the increase of the phase proportion of martensite and/or coldformingand/or precipitation hardening. In austenitic-martensitic steels, the0.2% technical elastic limit, the tensile strength, and the hardness areincreased in comparison to austenitic steels as a result of themartensite content. Rustproof austenitic-martensitic CrNi steels combinethe advantages of the austenitic steels and of the preferablysoft-martensitic steels.

The disadvantage of the aforementioned methods for increasing strengthresides in that generally they entail a deterioration of the toughnessproperties and thus in general of the transformation properties.Austenitic steels with TRIP/TWIP effect (transformation-inducedplasticity and twinning-induced plasticity) compensate this disadvantagein that one or several transformation-induced martensites or twinningare induced during coldforming. These effects cause a simultaneousincrease of the tensile strength and of the elongation at break so thatcoldforming properties are improved and energy absorption capacityincreases. For austenitic-martensitic steels, there are no solutionsdisclosed yet for eliminating this disadvantage and the loss oftoughness as strength is increased.

High-alloy austenitic-martensitic steels are rustproof steels [1] orhigh-manganese steels and obviously also LIP steels (light inducedplasticity) [2, 3, 4]. There is no information available yet in theliterature in regard to LIP steels. Comprehensive test results in regardto the TRIP/TWIP effect and its effects on the mechanical properties andthe energy absorption capacity are available only for high-manganesesteels [2, 3]. These high-manganese steels contain no chromium and arethus not corrosion-resistant and weather-resistant or slow to corrode.

The high-manganese steels have 0.2% technical elastic limits of 200 to450 MPA and tensile strengths of 780 MPa to 1,100 MPa and elongation atbreak between 39 and 47%. For example, a steel with 15% manganese andsilicon content of 4 to 2% and aluminum content of 2 to 4% exhibitsthese properties [1, 2]. The alloying range in which theaustenitic-martensitic steels with TRIP effect exist has been specifiedpartially for high-manganese steels but not for rustproof steels [3].

For a targeted utilization of the TRIP effect it is necessary that thechemical composition of the steels with TRIP effect is adjusted withregard to chromium and nickel equivalents. For austenitic steels thathave excellent coldforming properties this is disclosed in [7]. In thisconnection, the ferrite-forming effect of chromium, silicon, andaluminum is represented by the chromium equivalent and the austeniticstabilization effect of the elements manganese and nickel is representedby the nickel equivalent. In this connection, it was demonstrated thataluminum increases the Ms temperature and therefore has an effect on thenickel equivalent. In regard to the effect on Ms temperature, aluminumtherefore behaves reversely to the other alloying elements andaccompanying elements. Recent tests have shown that the effect ofaluminum on the Ms temperature is weaker than disclosed in [7].

Moreover, aluminum and silicon have a detectable positive effect on thepassivation behavior of rustproof steels and on the rust layer formationin weather-resistant steels and corrosion-resistant steels. At the sametime, these elements however can cause deterioration of the coldformingproperties and the surface quality of the products. This is adisadvantage when relatively large aluminum-containing andsilicon-containing oxide inclusions are preferably formed in steels.

The patents EP 1 0901 006 B1 [8], EP 1 006 706 B1 [9], and EP 0 031 800B1 [10] disclose ultra high strength steels whose tensile strengths areabove 2,200 MPa. These steels are originally austenitic steels that havebeen subjected to coldforming and subsequently have been subjected to anaging or precipitation hardening process. The high tensile strengths arethen achieved in the thus treated material. This coldformed material isvery brittle and can hardly be elongated. It is no longer designed for afurther coldforming process.

For evaluating the coldforming properties of steels, the product oftensile strength and maximum elongation can be utilized as acharacteristic value. The product of maximum elongation and tensilestrength for austenitic-martensitic steels is in the range above 20,000MPa % [3-5]. Despite relatively high tensile strength, the steels canstill be coldformed relatively well. The steels still have residualenergy absorption capacity. This means that in case of crash loading theaustenitic martensitic steels still have a satisfactorily highelongation buffer [3-5].

By means of the stacking fault energy of the austenite that is dependenton the chemical composition of the austenite, the differentstrength-increasing mechanisms can be affected in principle [2, 6].

One condition for the formation of transformation-induced α′ martensiteis that the micro-structure is comprised at least partially ofaustenite. Moreover, the austenite must be metastable in order to have acorrespondingly high tendency for forming transformation-inducedmartensite. For these reasons, for the chemical composition of thesteels an appropriate chromium equivalent and nickel equivalent arerequired. This means that in the chemical composition of the steels theferrite-stabilizing and austenite-stabilizing elements must be adjustedrelative to one another. For this reason, a modified chromium equivalentand a known nickel equivalent have been used in order to specify, asformulated in the claim, the range of existence oftransformation-induced α′ phase formation. Under these conditions, therequired chemical composition of the steel according to the inventioncan be determined.

LITERATURE

[1] Stahlschlüssel 2004, Verlag Stahlschlüssel Wegst GmbH

[2] Grässel, O., L. Krüger, G. Frommeyer, and L. W. Meyer, Intern. J.Plasticity 16 (2000)

[3] Frommeyer, G.: Published Application, DE 197 27 759 A1, pp.1391-1409

[4] Schröder, T.: Technische Rundschau, 1/2 (2006), pp. 48-52

[5] Bode, R. a.o.: stahl und eisen 8 (2004), pp. 19 to 26

[6] Martinez, L. G. u.a.: Steal research 63 (1992) 5, pp. 221-223

[7] Weiβ, A., H. Gutte, and P. R. Scheller: DE 10 2005 024 029 A1

[8] Uehara, Toshihiro: Letters Patent EP 1 091 006 B1

[9] Hiramatsu, Naoto and Tomimura, Kouki: Letters Patent EP 1 106 706 B1[9]

[10] Malmgren, Nils: Letters Patent EP 0 031 800 B1

The invention as defined in the independent claims therefore concernsthe problem of providing austenitic-martensitic lightweight steels withexcellent coldforming properties and with tensile strengths between 800to 1,200 MPa and elongation at break greater than 25%.

This object is solved by the invention in accordance with theindependent claims and advantageously the dependent claims.

The advantages achieved with the invention reside in particular in thatwith the lightweight steels according to the invention an improvement ofthe strength properties is achieved and, at the same time, the toughnessproperties remain at a relatively high level. These steels arecharacterized therefore by a good combination of high strength and, atthe same time, good toughness properties. Accordingly, these steels canstill be relatively well coldformed and have still a relatively highenergy absorption capacity.

The invention will be explained in the following preferred embodiments.

The lightweight steels according to the invention can be divided intotwo different steel types. The first steel type comprises rustprooflightweight steels with TRIP effect and with chromium content in thelimits of greater than 12.0 to 18%. The second steel type compriseslightweight steels with TRIP/TWIP effect and with chromium content ofmore than 0.5% and smaller than 12.0% that generally areweather-resistant and corrosion-resistant.

EXAMPLE 1

Preferably the inventive high-strength lightweight steel with TRIPeffect has a carbon content of 0.03%, a chromium content of 14.1%, asilicon content of 1.23%, a nickel content of 6.3%, a manganese contentof 7.94%, an aluminum content of 0.051%, and a niobium content of 0.5%,the remainder being essentially iron. The micro-structure of the steelis comprised primarily of metastable austenite and martensite. The steelexhibits a TRIP effect at room temperature. A high hardening capacity isobserved. The 0.2% technical elastic limit is approximately at 300 MPaand the tensile strength is at 890. The steel exhibits a maximumelongation of 45%.

EXAMPLE 2

Preferably, the inventive high-strength lightweight steel with TWIG/TRIPeffect has a carbon content of 0.04%, a chromium content of 0.52%, asilicon content of 1.5%, a nickel content of 2.1%, a manganese contentof 11.5%, and an aluminum content of 0.051%, the remainder beingessentially iron. The micro-structure of the steel is comprised ofmetastable austenite and martensite. The steel exhibits a TRIP/TWIGeffect. A relatively high hardening capacity is observed. The 0.2%technical elastic limit is at 310 MPa and the tensile strength is at1170 MPa and maximum elongation is at 31%.

In this way, the production of high-strength rustproof steels isachieved that form a passive layer on the surface. On the other hand, itis possible to produce high-strength steels that usually areweather-resistant or corrosion-resistant.

Since these steels are alloyed with chromium, silicon, and aluminum andpartially with nickel, they have increased resistance with regard tomaterial loss through rust. A variety of these steels can therefore beviewed as weather-resistant or corrosion-resistant. In particular steelswith chromium content of 10 to 12% have a distinct corrosion resistance.

The mechanical properties of the stainless steels according to theinvention with chromium content greater 12 and less than 18% arecomparable to the mechanical properties of rustproof soft-martensiticsteels inasmuch as there is still residual austenite in themicro-structure. The rustproof steels according to the invention have ingeneral in comparison to soft-martensitic steels low martensite and noferrite proportions in the un-transformed initial micro-structure. Onlyas a result of a TRIP effect in the process of coldforming, themartensite proportion in the steels according to the invention willincrease and reach values that are existing in soft-martensitic steelsin general already in the un-transformed initial state. Therefore, incomparison to the soft-martensitic steels, the steels according to theinvention generally have lower 0.2% technical elongation limits. At thesame time, the steels will harden strongly in the process of mechanicalloading and will reach almost the same or higher tensile strengths andhigh elongation at break. For this reason, these steels can still becoldformed well. Moreover, particularly in the rustproof CrNiMn steelsaccording to the present invention, the nickel content can be lowered incomparison to the commercially available soft-martensitic CrNi steels.This provides a cost-effective production of these steels. The steelaccording to the invention can be differentiated from steels as they aredisclosed in [7] by a lower nickel equivalent. Moreover, themicro-structure of the un-transformed initial state is comprised ofmartensite and austenite.

The advantage of the austenitic lightweight steels according to theinvention with chromium content between 0.5 and 12% relative tohigh-strength chromium-free lightweight steels resides in their weatherresistance and corrosion resistance. These properties are achieved inthe case of a tightly adhering rust layer. The strength and toughnessproperties of this group of steels according to the invention inindividual situations approach the excellent mechanical properties ofhigh-manganese TRIP/TWIP steels. These steels according to the inventionwith rust layer formation can also still be coldformed and still have arelatively high energy absorption capacity.

The austenite in the steels according to the invention is metastable. Bymeans of a mechanical treatment it is possible to affect themicro-structure of the austenite with regard to generating stackingfaults, twinning, and transformation-induced martensite, preferablytransformation-induced α′ martensite.

By employing alloy-technological measures, the formation of preferablytransformation-induced α′ martensite in an austenitic-martensiticmicro-structure is activated in the steel according to the invention.For this purpose, the nickel equivalent relative to the coldformableaustenitic lightweight steels [7] is lowered. The steels according tothe invention differ in this respect from the austenitic lightweightsteels that can be coldformed well.

In the austenitic-martensitic steel according to the invention, theindicated property potential is however achieved in the process ofmechanical loading as a result of transformation-induced martensiteformation and without after treatment. In this way, the steels accordingto the present invention differ in principle from the ultra highstrength steels as they are disclosed in [8, 9, 10]. The steel accordingto the invention can possibly have a chemical composition as observed inaluminum-containing CrNi steels [8, 10] as well as in those that containTi, Si, Nb, and V [9].

Manganese is alloyed in the steels according to the invention as anaustenite former and as a substitution element for nickel.

Titanium and niobium improve moreover the formation of austenitic finegrain and cause a fine martensite structure. Accordingly, these elementshave a positive effect on the mechanical properties. Moreover, niobiumand titanium effect binding of carbon and cause thus an improvement ofthe corrosion properties.

When the austenite of the austenitic-martensitic steels transforms,induced by mechanical loading, into ε and/or α′ martensite inside, aTRIP effect is observed. As a result thereof, the plastic deformationcapability and the tensile strength are increased. By twinning, theseproperty changes can be enhanced even more. A high hardening potentialis then observed. In contrast to the metastable austenitic steels withTRIP effect, austenitic-martensitic steels with TRIP effect have ahigher 0.2% technical elastic limit and higher tensile strengths.

The steels according to the invention differ from the known austeniticTRIP/TWIP steels in that the TRIP effect is induced not in an austeniticinitial micro-structure but in an austenitic-martensiticmicro-structure. The tensile strengths of more than 800 MPa are thusmainly the result of the already existing annealed martensite and of thetransformation martensite. Elongation at break of more than 25% iscaused primarily by the TRIP effect and thus the formation oftransformation martensite. Precipitation hardening or aging is notrequired in order to obtain the indicated mechanical properties.

In order to minimize the known negative effects of aluminum,metallurgical measures with regard to oxygen uptake of the melt and thusof the dissolved oxygen content as well as with regard to precipitationof such inclusions are required. The dissolved oxygen content in themelt should therefore not surpass a value of 0.003% in the steelaccording to the invention.

Aluminum is special with regard to its alloying effect. As aferrite-stabilizing element it has an effect on the chromium equivalentas expressed in the relationship 1 of claim 1. The effective factor ofaluminum on the nickel equivalent in the relationship 2 indicated inclaim 1 has been set to −0.2.

1. High-strength austenitic-martensitic lightweight steel having atensile strength greater than 800 to 1.200 MPa and an elongation atbreak greater than 25%, characterized in that the steel has a chromiumcontent of greater than 0.5% and less than 18%, a silicon content ofgreater than 1% and less than 4%, a manganese content greater than 2.5%and less than 30%, and an aluminum content greater than 0.05% to 4%, andis within an alloying range that is determined by the coordinates offour points (Cr_(equ)=2; Ni_(equ)=2), (Cr_(equ)=2; Ni_(equ)=24),(Cr_(equ)=20; Ni_(equ)=10), and (Cr_(equ)=20; Ni_(equ)=6.5), wherein thechromium and nickel equivalents are calculated with the relationships 1and 2Cr_(equ)=% Cr+% Mo+1.5% Si+0.5% W+0.9% Nb+4% Al+4% Ti+1.5% V   (1)Ni_(equ)=% Ni+30% Cr+18% N+0.5% Mn+0.3% Co+0.2% Cu−0.2% Al   (2) basedon the chemical composition of the steel, wherein the values are to beapplied in % by weight and wherein the remainder is essentially iron andother accompanying elements (P, S) of steel and is coldformable. 2.Lightweight steel according to claim 1, characterized in that the nickelcontent is from 0 to 10%, the niobium content is from 0 to 1.2%, thecarbon content is from 0.01 to 0.2%, the nitrogen content is from 0 to0.1%, the copper content is from 0 to 4%, the cobalt content is from 0to 1%, the molybdenum content is from 0 to 4%, the tungsten content isfrom 0 to 3%, the titanium content is from 0 to 1%, and the vanadiumcontent is from 0 to 0.15%, the oxygen content dissolved in the steel isless than 0.003%, and the remainder is essentially iron.
 3. Lightweightsteel according to claim 1, characterized in that the carbon content is0.03%, the chromium content is 14.1%, the silicon content is 1.23%, thenickel content is 6.3%, the manganese content is 7.94%, the aluminumcontent is 0.051%, the niobium content is 0.5%, and the remainder isessentially iron.
 4. Lightweight steel according to claim 1,characterized in that the carbon content is 0.04%, the chromium contentis 0.52%, the silicon content is 1.5%, the nickel content is 2.1%, themanganese content is 11.5%, and the aluminum content is 0.051%, and theremainder is essentially iron.
 5. The lightweight steel according toclaim 1 as a material for hot-rolled and/or cold-rolled sheet steel,bands, and pipes.
 6. The lightweight steel according to claim 1 as amaterial for non-flat products, non-flat semi-finished products, wire,cold massive formed parts and fasting elements.
 7. The lightweight steelaccording to claim 1 as a material for crash-loaded components andreinforcing structural components.
 8. The lightweight steel according toclaim 1 as a material for wear parts.
 9. The lightweight steel accordingto claim 3 wherein the material is subjected to a heat treatment beforebeing coldformed.
 10. The lightweight steel according to claim 1 as amaterial for rustproof parts.
 11. The lightweight steel according toclaim 1 as a material for weather-resistant and corrosion-resistantparts.